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Basic Electronics » USB cable specifications

August 28, 2013
by sask55
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Where can I find a data sheet or general information on the USB to serial cable adapter that is supplied with a Nerdkit. Specifically how much current, if any, will the adapter data lines sink in the low logic level state? Also what would be the current that could be sourced from the data lines in the high logic level state?

August 28, 2013
by Noter
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The datasheet for the PL2303 can be found in several places and here is one, http://www.stkaiser.de/anleitung/files/PL2303.pdf. I took a quick look but didn't see any ratings for the data pins in terms of current so I guess you will have to experiment and see where the voltage starts dropping and consider that the max. I am curious as to what you want to drive with the usb data lines?

August 28, 2013
by BobaMosfet
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sask55--

Current for USB, depends on revision and speed (generally speaking). And comes in 3 flavors: 100mA, 200mA and 500mA. If you're in windows, go look in the Device Manager for the USB port with your USB<->Serial adapter and see how much current is being output (it will tell you the max, as well).

BM

August 28, 2013
by sask55
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Notor

I am driving optical isolator chips.

I have been using optical isolator chips as a safety measure. Using them makes it is very unlikely that I will damage any components on the PC or the USB with misconnections or bad design. I have noticed that most commensally available boards ,motor controllers, and switching circuits, even some sensing circuits make use of optical insulators. I have some projects dealing with relatively higher voltages, up to 240 AC and high amperage capabilities up to 50 amps I prefer to think no matter what it is unlikely I will fry my USB or mother board. on the PC.

I have developed a circuit that has bean working flawlessly for me on a number of boards. After looking over the optocoupler setup on Ralph’s motor controller board, it occurred to that there may be a better way to do the circuit. If the USB adapter will sink enough current to activate the LED on the optocoupler I could simplify the circuit, remove the 4714 inverter chip as well as a number of resistors. I don’t know why I never thought of this approach before it is so simple. The circuit boards on my long lasting mill project are very congested anything I can do to reduce the number of components would help me with the PC board layouts.

This entire change that I am considering is dependent on being able to sink current to the USB data TX line when it is in the low logic state rather then sourcing current when it is in the high logic state. I would also have to make a similar change to the connections to the micro. This would invert the logic and eliminate the need for some components.

August 28, 2013
by Noter
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I've put led's on the tx/rx lines in the past as activity indicators but I used transistors to drive the them. One thing to keep in mind is the tx line from the usb adapter is 3.3v. Maybe the MAX3250 will do the trick?

August 28, 2013
by sask55
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Thanks Noter

I took a quick look at the MAX3250. It may work but it is certainly much more involved then the circuit that is working well for me now. I am looking to simplify the isolation if possible down to two optocouplers and two resistors.

I realize that the TX line is 3.3V. Is the USB power also 3.3 volts? In order to maintain total electrical isolation between the Micro and the USB I would be using the USB power line (red wire) on the anode and the TX line on the cathode side of the optocoupler internal diode. There will be no connection to the micro data pins, ground or power from the USB, maintaining total isolation.

The question is will the TX line from the adapter sink current supplied by the USB power line through a current limiting resistor and the optocoupler LED when the TX line is low? Or at the very least I would like to know, will it do damage to the adapter to try this? To put it another way can I put 3.3V to the TX wire of the USB adapter and will it sink a small amount of current when it is in the low voltage logic state without any problems?

I believe that this will work well on the micro side of the isolation because the pins on the micro will sink current. When a pin configured as an output pin is low (zero volt state)it will sink current if it is suplied from the 5V micro power supply.

Basically I am just thinking about inverting the data signals both ways without the use of the inverter chip as I am doing now.

August 29, 2013
by Noter
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USB power is 5v and I don't think it will hurt the adapter to give it a try but best to have a spare just in case. Bluetooth wireless might be a good alternative. You could go with a HC05 master on the PC side and a 4 pin HC06 slave on the PCB.

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